Fecapentaenes are potent human fecal mutagens derived from anaerobic bacteria and are therefore potential candidates as human colon carcinogens. Several in vitro studies by other investigators have demonstrated potent mutagenic effects of fecapentaene-12 (FP-12) in bacterial cells, low cell transforming activity in vitro in some cells and mammalian mutagenic activity. When earlier studies provided no evidence for the carcinogenicity of FP-12, we tested the corresponding diacetate ester; despite its greater solubility and lipophilicity, this agent also failed to produce tumors in the skin painting studies employed. Since epidemiological findings of fecapentaene levels in human stools have shown no increased FP levels in colon cancer patients, this project has been phased out.